SECTION 5.3 Physics of Plate Movement

What do we know? What are some clues to the movement of the Earth’s plates? What is under the plates? What are some forces that act on the Earth’s plates?

Convey the Concept Describe how a conveyor belt works. Make a list of where conveyor belts are used. What happens to objects places on a conveyor belt?

Plate Tectonics 50 years after Alfred Wegener proposed the theory of plate tectonics, it was finally accepted. Because there has been evidence collected about the movement of plates, we know:  How fast plates move  Which direction they move in  And how they interact at boundaries. But one question still remains: What is the driving force that moves the plates? FPlate_tectonics&ei=NH9kVePHL8rwoASq0YDACA&bvm=bv ,d.cGU&psig=AFQjCNGOpmXqlzSetXz- bZiTJgsyjyLfjQ&ust= &cad=rja

Convection in the Mantle This question is somewhat answered because we know that the driving force of the movement has something to do with the movement of mantle. This is where convection occurs.  Convection- heating of the rock in the partially melted zone of the mantle causes that material to move in a circular motion. Convection occurs in circular units of movement called convection cells. Most scientists agree that convection causes plate movement but there is no agreement about how convection happens.

Convection in the Mantle There are two basic models for convection both supported by strong evidence.:  One model shows that convection happens mainly in the asthenosphere. (Right side of Figure 5.15).  The other model shows that the convection cells are much larger and they include the entire mantle. (Left side of Figure)

Models for Plate Movement There is disagreement about how convection moves plates. One model shows that the plates are carried along by the movement of convection. (Passengers on giant conveyor belts)  Where convection pushes material upward, divergent boundaries form.  Where convection goes downward back into the mantle, convergent boundaries form. Another model shows that the plates themselves have an active role in convection.  Each plate is thought to be a convection cell.  With the plate being cooler and more dense than the asthenosphere below, it tends to sink.  This sinking, pulls the rest of the plate down which helps create the circular movement of convection.

What have we learned? 1. What are two models for convection in the mantle? 2. If convection cells in the mantle are like conveyer belts, what are the plates like? 3. Which model do you think best explains how convection moves the plates? Explain the reasons for your choice. 4. What are the similarities and differences between the two major models explaining how convection causes plate movement?